1. Field of the Invention
The present invention relates to a system and method for mapping video inputs to addresses for color look-up tables, and more particularly, to a system and method for mapping a pixel input to an address of an entry in a window specific color look-up table in accordance with a window-specific display mode.
2. Description of the Prior Art
The image refresh system of raster displays often includes a so-called video color look-up table (also called a color table or color map). In such systems, the color of each pixel in an image is coded by a value which is not usually routed directly to the digital-to-analog converter. Instead, this value is used as an index into the look-up table. A value in the table entry indexed by the pixel value is then used to control the display color for that pixel. Since many color applications do not require all of the available colors in a single picture, the look-up table typically contains only the colors necessary to render the image. This technique thus saves memory space in the color display device.
FIG. 1 illustrates the operation of a prior art color look-up table of the type just described. As shown, a part of the memory is organized into frame buffer 100 where color information for each pixel 102 is stored. The color information for pixel 102 in frame buffer 100 represents an index 104 to a particular color in the color look-up table 106. In the example, index 104 has a value of "67" which points to a table entry 108 in color look-up table 106 at address "67". As shown, the table entry value 108 indexed by the index 104 with value "67" actually contains a twelve-bit value "100110100001" which represents the color information for a particular pixel of the display screen. As shown, this table entry value 108 is actually an aggregate of three 4-bit values for red, green, and blue as shown at 110. Each color component value at 110 is used to control one or more color guns 112 which actually render the specified color to a pixel 114 at location (X.sub.o,Y.sub.o) on the CRT display. The above-described color look-up operation is repeated for every pixel on the CRT display until the whole image is rendered.
A single set of color maps is typically addressed for all pixels of the display screen. In other words, in a multi-window environment, each window typically does not have its own set of colors. This is the case because the ability to maintain an independent color look-up table for each window is burdensome for the Central Processing Unit (CPU). Thus, when refreshing images in different windows, window-specific pixel values must be converted to global coordinates for use as indices to the color look-up table containing the actual red, green, and blue values. This is also burdensome for the CPU. It is desired that the color look-up tables be made window-specific to avoid such extra processing. Also, by making the color look-up tables window-specific, independent display modes for each window will be made possible.
The use of different display modes for each window makes possible many new possibilities for the presentation of computer graphics images. For example, in a blending mode one window may allow overlaying of an image over another image in that window while another window may compare respective images. However, such calculations must be better supported by a dedicated piece of hardware so that the CPU can be relieved from these multiple display tasks. Otherwise, system performance degrades too much to make window-specific display modes practical for use in graphics display systems.
Thus, a need exists for a dedicated piece of hardware that supports independent display modes and red, green, and blue color look-up tables for each window in a multi-window environment without burdening the CPU. Such a device should be able to keep track of window-specific attributes and to refresh the window images according to the attributes. Such a device should also be able to map the window-specific pixel values into addresses for window-specific color look-up tables. The present invention has been designed to meet these needs.